LRD doesn't have edit history, but is it hidden in xmp files that can be seen by LRC later?
No, it's not hidden anywhere - it doesn't exist.
If I create Collections with the travel LRC cataglog on the laptop, when it comes time to export my new travel photos, will the Collections be exported together with the new photos?
Is it easier to retain the Collections made on the road with LRC or with LRD?
Yes, if you use File > Import from Another Catalog on the desktop, after doing a File > Export as Catalog on the laptop. Try it - it's quite obvious, and this LrC-only workflow retains everything that you do in LrC.
If you wish to use LrD on the road, Collections will simply sync into your LrC catalog. So that can be less work. But then you're really going to need to use LrD, you'd not have any editing history and smart collections, and you'd have to see if you can get your head around using one app on the laptop, a different one on the desktop.
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